It is sometimes necessary to stop fluid flow through a pipeline, for example a water pipeline, without the assistance of a valve. For example, a section of pipeline may not be controlled by an upstream valve that will allow fluid flow to be shut-off during pipeline repairs. Alternatively, the pipeline may include a valve, but that valve may be broken or otherwise not fully functioning. In either case, plugging may be used to temporarily shut-off the fluid flow such that needed repairs or other servicing operations can be made to a section pipeline.
In one type of plugging operation, two halves line-stopping sheath or sleeve are placed around the pipe at a point along the pipeline where fluid flow is to be shut-off. The two halves of the sheath are typically bolted together. The upper half of the sheath supports a spigot assembly (i.e. a short piece of pipe welded to a flange), to which a temporary valve is then attached. An operator then extends a tapping machine through the temporary valve and the spigot assembly and taps an aperture into the wall of the underlying pipe. The temporary valve is used to prevent fluid escape after the tapping machine is removed.
Next, a plugging machine is used to extend a plug through the temporary valve (which is again opened), the spigot assembly, and the aperture through the pipe wall. Once the plug is in place within the inner walls of the pipe, the fluid flow is stopped and repairs or other service work can be performed downstream of the plug. In some instances, a bypass may be put in place around the length of pipe under repair.
The pipe normally used to fabricate the each spigot assembly is relatively expensive, especially if stainless steel pipe is used. For example, ⅜ to ½ inch thick stainless steel pipe typically costs on the order of $200 dollars per linear foot, which significantly adds to the cost of each spigot assembly and consequently to the cost of the overall line-stopping sheath assembly. Moreover, the flange portion of the spigot assembly may be fabricated from stainless steel, which adds further expense and weight to the sheath assembly.
It would be highly advantageous to reduce the cost of each spigot, and in turn the cost of each spigot-flange assembly, as well as the overall line stopping assembly. Furthermore, reducing the weight of the spigot-flange assembly would also realize significant advantages.